High gain and low offset comparators are usually designed with the first gain stage very high. In order to do this, the output of the first gain stage often uses a cascode configuration for the active load.
However, when the inputs of the comparator are not balanced, that is, they are far from the switching point of the comparator, the voltage output of the comparator is either close to the supply or close to the ground. In this state, the output devices of the comparator are pushed close to, or into, the triode region. Since the parasitic capacitances of the output devices (e.g., at the gate-drain (GD)) can be much higher in the linear region than in saturation, the comparator can lose time charging these greater capacitances, reducing the overall speed of the comparator.
Accordingly, comparators designed for high gain and low offset can have a speed limitation when output devices of the first internal stage go into the linear region and the inputs to the comparator are unbalanced. This is particularly true when the output devices of the first gain stage are “cascoded.” As a result, the comparators may be slower than desired for some applications.